1. Field of the Invention
The present invention generally relates to the field of semiconductor devices, and more particularly to a semiconductor device with an epitaxial stricture and a fabrication method thereof. 2. Description of the Prior Art
As semiconductor devices switching speeds continue to increase and operating voltage levels continue to decrease, the performances of metal-oxide-semiconductor filed effect transistors (MOSFETs) and other types of transistors, such as bipolar junction transistors, need to be correspondingly improved. Currently, along with the development of the MOSFETs, one of the main goals in the industry is to increase the carrier mobility so as to further increase the operation speed of the MOSFETs.
In order to improve the device performances, crystal strain technology has been developed. Crystal strain technology is becoming more and more attractive as a means for getting better performances in the field of MOS transistor fabrication. Putting a strain on a semiconductor crystal alters the speed at which charges move through that crystal. Strains make the MOS transistors work better by enabling charge carriers, such as electrons or holes, to pass through the silicon lattice of the channel more easily. Currently, attempts have been made to use a strained silicon layer as a part of MOS transistors in which an epitaxial silicon germanium (SiGe) structure or an epitaxial silicon carbide (SiC) structure is formed. In this type of MOS transistor, a biaxial tensile strain is induced in the epitaxy silicon layer due to the difference in lattice constants between SiGe or SiC and Si. As a result, the band structure is altered, and the carrier mobility is increased.
However, due to the continuous shrinkage in the size of the semiconductor devices, the aspect ratio of the epitaxial layer also gets higher, which often incur unwanted defects, such as void defects in the epitaxial layer. These defects inside the epitaxial layer reduce the stress required to be imposed onto the corresponding channel region. As a result, how to prevent the formation of the defects inside the epitaxial layer is an important issue.